648 SPECIAL SENSES 



supplying the inner, or nasal mathematical half of the retina, from a 

 vertical line passing through the macula lutea. Fibres also pass to the 

 left eye, supplying the outer, or temporal half of the retina. The ma- 

 cula lutea, then, and not the point of entrance of the optic nerve, is in the 

 true vertical line of division of the retina. 



With the exception of a few grayish filaments, the fibres of the optic 

 tracts and the optic nerves are of the ordinary medullated variety and 

 present no differences in structure from the general cerebro-spinal 

 nerves. 



The optic commissure is covered with a fibrous membrane and is 

 more .resisting than the optic tracts. The optic nerves are rounded and 

 are enclosed in a double sheath derived from the dura mater and the 

 arachnoid. They pass into the orbit on either side and penetrate the 



sclerotic at the posterior, inferior and internal 

 portion of the globe. As the nerves enter 

 the globe they lose their coverings from the 

 dura mater and arachnoid. The sheath de- 

 rived from the dura mater is adherent to the 

 periosteum of the orbit at the sphenoidal 

 fissure, and when it reaches the globe, it fuses 

 with the sclerotic coat. Just before the 

 Fig. 165. -Diagram of the de- nerves penetrate the globe, they each present 

 cassation of fibres at the optu a well-marked constriction. At the point of 



commissure. . . . i i 



penetration there is a thin but strong mem- 



The dotted lines show the four , ^. , f ,. . ,. 



directions of the fibres. brane, presenting a number of perforations for 



the passage of the nervous filaments. This 



membrane, the lamina cribrosa, is in part derived from the sclerotic, and 

 in part, from the coverings of the individual nerve-fibres, which lose their 

 investing membranes at this point. In the interior of each eye there 

 is a little mammillated eminence, formed by. the united fibres of the 

 nerve. The retina, with which the optic nerve is connected, will be 

 described as .one of -the coats of the eye. 



In the centre of the optic nerve is a minute canal, lined with fibrous 

 tissue, in which are lodged the central artery of the retina and its corre- 

 sponding vein, with a delicate nervous filament from the ophthalmic gan- 

 glion. The vessels ^penetrate the optic nerve J to f of an inch (8.5 to 

 19.1 millimeters) behind the globe. The central canal does not exist 

 behind these vessels. 



General Properties of the Optic Nerves. There is little to be said 

 regarding the general properties of the optic nerves, except that they 

 are the only nerves capable of conveying to the cerebrum the special 

 impressions .of sight and are not endowed with general sensibility. 



